Upper boundary of argillic horizon and evidence of illuviation?

To my knowledge is not specified where evidence of illuviation/high extensibility must be observed in order to meet the requirements for an argillic.

Is it correct to say that the upper boundary of the argillic corresponds to the depth at which the necessary clay increase from eluvial->illuvial occurs regardless of the described morphology of the horizon (i.e. whether first subhorizon has t subscript/clay films/bridging/oriented clay in section)?

Is the description from the "Top of horizon" special problem section in the argillic definition in Soil Taxonomy (2nd edition) the current official stance on this topic?

Andrew Brown
USDA-NRCS Soil Scientist
SWSON MLRA Office - Sonora, CA